Galvanic cell employing iron cathode and method of producing galvanic cathode having activated iron surface



United States Patent 3,477,876 GALVANIC CELL EMPLOYING IRON CATH- ODE AND METHOD OF PRODUCING GAL- VANIC CATHODE HAVING ACTIVATED IRON SURFACE Paul R. Juckniess and Robert D. Blue, Midland, Mich, as-. signors to The Dow Chemical Company, Midland,

Mich., a corporation of Delaware No Drawing. Filed June 13, 1966, Ser. No. 556,857 Int. Cl. H01m 15/04 U.S. Cl. 136--83 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for producing an improved iron cathode useful as the cathodic electrode in a water depolarized magnesium battery and more particularly relates to a process for producing an improved battery cathode having an activated iron surface.

Many attempts have been made to utilize the electromotive potential of metallic magnesium as the basis for a primary cell having an attractive ratio of output energy to weight. A suitable cathode for such a cell, however, needs the characteristics of relatively high physical strength, a high surface to volume ratio and the ability to produce high initial power. Until now only relatively expensive cathodes, such as metals plated with platinum, for example, have been suitable for use in water depolarized magnesium batteries. Such high cost cathodes have prevented the use of such batteries in many applications.

It is an object of this invention to provide a process for coating a relatively inexpensive substrate such as iron with an activated iron surface. A further object is to provide an improved yet inexpensive cathode having an activated iron surface and suitable for use with a magnesium anode to provide a water depolarized magnesium'battery. These and other objects and advantages of this invention will become apparent from a reading of the following detailed description. I

It has now been discovered that an improved galvanic cathode having an activated iron surface may be prepared by plating iron onto a conductive substrate froman electrolytic bath containing ferrous ions provided that such electrolysis is conducted at a temperature below about C., at a current density of at least 0.25 amp/in. of actual surface area, preferably 0.75 amp/in. of actual substrate surface area, and at a pH of between about 0.5 =and 3.5. The surface obtained by employing the process of this invention is composed of an activated form of iron in a finely divided, particulate form appearing, under magnification, to be bright nodules of iron. The particles which compose the activated iron surface show sufiiciently high activity, immediately after preparation, to decompose water and to be pyrophoric when dried in contact with 3,477,876 Patented Nov. 11, 1969 ice air. A cathode prepared in this manner, when employed in a water depolarized battery with a magnesium anode produces high initial power and produces performance characteristics which equal or exceed those of the platinum coated cathode.

As the substrate for the cathodes of this invention, any metal which will take an iron plate is suitable but a relatively pure iron which will give a bright pickled surface is usually preferred since it is readily available, relatively inexpensive and is easily plated with a strong, adherent coating of activated iron. The substrate may be of any desired configuration but flat plates, perforated plates and wire screens are usually most desirable. Some advantage is found in employing a perforated metal plate or a wire screen as the substrate since better adhesion of the activated iron coating is usually obtained.

The plating of the activated iron onto the substrate is accomplished by employing a standard ferrous ion-containing bath having a pH of between about 0.5 and about 3.5, preferably between about 1.0 and about 2.5. Such bath usually contains a ferrous ion source 'such as FeCl and a watersoluble salt such as CaCl to increase bath conductivity. In order to produce the activated iron coating of this invention, it is essential that the final plating be conducted at a temperature of from the freezing point of the bath to about 15 C., preferably from about 0 C. to about 6 C. and at a current density of at least 0.25 amp/in. of actual surface. It is particularly desirable to employ a current density of from about 0.75 amp/in. to about 2.0 amps/in}. Current densities of 3 amps/in. or more may be employed but no additional advantage is achieved.

In those instances where a solid strip or sheet of iron or other solid metal substrate is employed on which it is ditficult to obtain good uniform adherence of the activated iron, it is advantageous to first plate such substrate with an ordinary iron coating by conducting suchplating at a higher temperature, e.g. 4050 C. and at a lower current density, e.g. about 0.125 to 0.25 amp/in}. The activated iron may then be plated over the thus-obtained iron plate. Still better adhesion is obtained when the active iron is first plated at a current density in the lower portion of thedesired range then the current density is gradually increased to the upper portion of the desired range. No advantage is gained by employing this special plating step if the substrate is a wire screen rather tharra solid metal plate.

The activated iron cathode prepared in accordance with this invention, when coupled to a magnesium anode in a saline water environment shows an initial current density of the same magnitude of the typical platinum coated cathode and an extended performance substantially equivalent to a platinum coated cathode. In general, the cathodez-of this invention when coupled to a magnesium anode in as. saline water environment will produce a voltage of from 0.3 to 0.5 volt at a current density of about 50 milliamperes (ma) per square inch depending on the electrode spacing and cell environment employed.

a It should be noted that, within the context of this invention, the aqueous electrolyte may be substantially any aqueous medium providing the water necessary for the chemical reaction. Any electrically conductive aqueous solution may therefore be employed in the cell, although inexpensive electrolytes such as sea water or brine are usually most desirable from an economic standpoint. However, special electrolytes with or without depolarizers may be employed.

For the anode herein, pure magnesium or magnesium alloys may be used and the term magnesium as used herein refers to both pure magnesium and to high magnesium content alloys containing at least about 70 percent magnesium.

The following examples are provided to more fully illustrate the invention but are not to be construed a limiting to the scope thereof.

EXAMPLE 1 A 4 inch x 4 inch fiat plate of SAE 1010 steel .010 inch thick was cleaned by washing with soap and hot water then rinsing with acetone, pickling in concentrated aqua regia, and finally washing in distilled water. The clean steel was pretreated in an aqueous bath having a pH of 1.0 and containing 190 gm./l. FeCl and 95 gm./1. CaCl by plating at a bath temperature of 42 C. and a current density of 0.5 amp/m The temperature of the plating bath was then lowered and the sample was plated according to the following schedule:

Current Bath Current Time density temperature, (amps) (minutes) (amps/in?) C. pH

The active iron coating on the plate was uniform and not blistered. When examined under the microscope it appeared to have a fine nodular surface and when first removed from the plating bath the surface was sufficiently active to decompose water with the release of hydrogen.

When the active iron cathode was coupled to a magnesium alloy anode containing about 3 weight percent Al and 1 weight percent Zn, spaced about inch from the anode and immersed in 3 weight percent NaCl solution, the cell produced a voltage of 0.45 volt at a current drain of 50 ma. per square inch.

EXAMPLE 2 As substrates in three runs, 3 inch x 3 inch screens were employed which were made from 0.17 inch Wire woven at 40 mesh spacing. After cleaning as in Example 1, these screens were immersed in aqueous plating baths having a pH of 1.0 and containing 190 gm./l. of FeCl and 95 gm./l. of CaCl During the plating operation, the temperature of the baths was maintained between 0 C. and 6 C. and the current density was held constant at 1.6 amps/in. of actual surface area of the screen being plated. One screen substrate was plated for 8.8 minutes (cathode A), one for 4.4 minutes (cathode B) and one for 2.2 minutes (cathode C).

In each instance a uniform adherent coating of activated iron was produced which, under magnification, appeared to be composed of finely divided, bright nodules of iron evenly distributed over the surface of the wire screen. Such activated iron cathodes were electrically connected to a magnesium alloy anode (containing about 3 weight percent Al and about 1 weight percent Zn, balance substantially magnesium) and immersed in a 3 weight percent solution of NaCl in water. The voltages produced at a drain of 50 ma./in. were as follows:

EXAMPLE 3 A 3 inch x 3 inch screen cathode, made in a similar manner to cathode B in Example 2, was coupled to two magnesium alloy anodes 3 inches x 3 inches x .095 inch thick, containing 3 weight percent Al and 1 weight percent Zn and spaced inch from the cathode.

The electrodes were immersed in synthetic sea water and discharged through a 1.5 ohm resistance. The initial cell voltage was 0.46 volt. This gradually decreased to 0.40 volt after seven days of continuous operation at which time the magnesium anode was almost completely consumed. The cathode screen was then cleaned, coupled to two new magnesium alloy anodes and again immersed in synthetic sea water. This cell performed in a similar manner as before and yielded a voltage in excess of 0.40 volt for seven days.

EXAMPLE 4 In a control run to illustrate and compare a cathode of the known art, a 2 inch x 2 inch iron screen was cleaned by the procedure of Example 1, placed in a plating bath of the same composition as the bath in Example 1 and plated at 0.714 amp/in? for 10 minutes at 23 C. When coupled to a magnesium alloy anode containing about 3 weight percent Al and about 1 weight percent Zn in a 3 weight percent aqueous NaCl solution, the voltage produced was 0.27 volt at a discharge rate of 25 ma./sq. in. and 0.18 volt at 50 ma./sq. in.

EXAMPLE 5 In another control run, conducted in the same manner as Example 4, a 2 inch x 2 inch nickel plated screen was plated with nickel for 30 minutes at 0.074 amp/in. and 23 C. When coupled to a magnesium alloy, as in Example 4, and placed in a 3 weight percent aqueous NaCl solution the cell produced a voltage of 0.15 volt at a discharge rate of 25 ma./in.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. A process for producing an improved iron cathode for use in a water depolarized magnesium battery which comprises contacting an electrically conductive substrate with an electroplating bath containing ferrous ions and having a pH of between about 0.5 and 3.5 and conducting the 'electrodeposition of activated iron on said substrate at a current density of at least about 0.25 amp/in. and at a bath temperature of between the freezing point of said bath and about 15 C.

2. The process of claim 1 wherein the electrically conductive substrate is a metal.

3. The process of claim 1 wherein the electrically conductive substrate is a relatively pure iron capable of giving a bright pickled surface.

4. The process of claim 1 wherein the bath temperature is between about 0 C. and about 6 C.

5. The process of claim 1 wherein the pH of the bath is between about 1.0 and about 2.5.

6. The process of claim 1 wherein the electroplating is conducted at a current density of at least about 0.75 amp/in. of actual surface of the substrate being plated.

-7. The process of claim 1 wherein the electroplating is conducted at a current density of between about 0.75 and 2.0 amps/in. of actual surface of the substrate being plated.

8. The process of claim 1 wherein the substrate is a wire screen.

9. In a galvanic cell comprising a metallic magnesium anode, an aqueous electrolyte and an electrically conductive cathode, the improvement in combination therewith which comprises employing a cathode having an activated iron surface, said iron surface, immediately after preparation, being characterized by being pyrophoric in dry air and decomposing water when in contact therewith and which, under magnification is in the form of finely divided, particulate, bright nodules of iron.

10. The galvanic cell of claim 9 wherein the cathode is a wire screen having said activated iron surface.

References Cited UNITED STATES PATENTS 6 3,036,141 5/1962 Goldenberg et a1. 136-100 3,256,504 6/1966 Fidelman 136--100 XR 3,305,401 2/1967 Aulin 136-420 3,345,212 10/ 1967 Schweitzer 136-25 5 WINSTON A. DOUGLAS, Primary Examiner Wh'tfi 1d t al. 204-48 A XR A. SKAPARS, Asslstant Exammer Beechlyn 136-100 Trask 204'48 U'S' X'R' Poor 204-48 10 136-100, 101; 204-48 

